1. Field of the Invention
This invention relates to offshore operations conducted from a floating vessel. More specifically, this invention relates to an improved marine riser system for use in drilling oil wells and similar boreholes from floating vessels.
2. Description of the Prior Art
In recent years the search for oil and gas has extended into increasingly deeper waters. Economic considerations and physical limitations frequently militate against the use of bottom supported platforms in very deep water. Therefore, most offshore drilling in deep water is conducted from a floating drilling vessel which supports the drill rig and derrick and associated drilling equipment.
A riser pipe is normally used to interconnect the floating vessel and the wellhead. A drill string extends from the floating vessel, through the riser and into the wellhead located on the sea floor. The riser pipe serves to guide the drill string into the wellhead and to conduct returning drilling fluid back to the vessel during drilling operations in the annulus between the riser pipe and drill string. The marine riser is presently regarded as the limiting element in floating drilling operations since the weight of the riser pipe and the stresses within the pipe increase with water depth. Adding to the stress on the riser pipe are bending moments caused by the action of wind, wave and sea currents on the riser and by movements of the drilling vessel.
To counteract riser stress, riser tensioning devices are normally mounted on the drill ship. These tensioning devices apply a constant tensile force to the top of the riser pipe, thereby reducing bending stresses on the riser. The use of flexible joints placed intermediate the ends of the riser has also been suggested to increase riser flexibility. However, both riser tensioning devices and flexible joints have their limitations as to the amount of riser stress which they can relieve.
In ordinary drilling using conventional risers, the riser pipe is about 17 to 20 inches in diameter. The relatively large diameter of the riser pipe is necessary to permit the drill bit or other large diameter tools used in connection with setting the casing to pass through the riser pipe. However, if a smaller diameter riser (12 to 15 inch inside diameter) can be used, the overall weight of the riser would be less, significantly reducing stress on the riser pipe.
The main problem with a small diameter riser is that it has to be retrieved every time it becomes necessary to run casing or tools into the well which have a diameter or width larger than the inside diameter of the riser. Complete retrieval of the riser is called a riser trip and each such trip can take from two to twenty days depending on water depth, weather conditions and other factors. If two or three riser trips are required during a drilling operation in deep water, as much as 40 days of expensive rig time can be lost. Thus, there is a need for a deep water drilling system which permits the use of small diameter risers without the concomitant loss of rig time caused by riser trips.